The Brain That Changes Itself

process, Merzenich invoked the ideas of Donald O. Hebb, a Canadian behavioral
psychologist who had worked with Penfield. In 1949 Hebb proposed that learning
linked neurons in new ways. He proposed that when two neurons fire at the same
time repeatedly (or when one fires, causing another to fire), chemical changes
occur in both, so that the two tend to connect more strongly. Hebb's concept —
actually proposed by Freud sixty years before — was neatly summarized by
neuroscientist Carla Shatz: Neurons that fire together wire together.
Hebb's theory thus argued that neuronal structure can be altered by experience.
Following Hebb, Merzenich's new theory was that neurons in brain maps develop
strong connections to one another when they are activated at the same moment
in time. And if maps could change, thought Merzenich, then there was reason to
hope that people born with problems in brain map-processing areas — people
with learning problems, psychological problems, strokes, or brain injuries —
might be able to form new maps if he could help them form new neuronal
connections, by getting their healthy neurons to fire together and wire together.
Starting in the late 1980s, Merzenich designed or participated in brilliant studies
to test whether brain maps are time based and whether their borders and
functioning can be manipulated by "playing" with the timing of input to them.
In one ingenious experiment, Merzenich mapped a normal monkey's hand, then
sewed together two of the monkey's fingers, so that both fingers moved as one.
After several months of allowing the monkey to use its sewn fingers, the monkey
was remapped. The two maps of the originally separate fingers had now merged
into a single map. If the experimenters touched any point on either finger, this
new single map would light up. Because all the movements and sensations in
those fingers always occurred simultaneously, they'd formed the same map. The
experiment showed that timing of the input to the neurons in the map was the
key to forming it — neurons that fired together in time wired together to make
one map.
Other scientists tested Merzenich's findings on human beings. Some people are
born with their fingers fused, a condition called syndactyly or "webbed-finger
syndrome." When two such people were mapped, the brain scan found that they
each had one large map for their fused fingers instead of two separate ones.
After surgeons separated the webbed fingers, the subjects' brains were remapped,
and two distinct maps emerged for the two separated digits. Because the fingers
could move independently, the neurons no longer fired simultaneously,
illustrating another principle of plasticity: if you separate the signals to neurons
in time, you create separate brain maps. In neuroscience this finding is now
summarized as Neurons that fire apart wire apart — or Neurons out of
sync fail to link.
In the next experiment in the sequence, Merzenich created a map for what might
be called a nonexistent finger that ran perpendicular to the other fingers. The